UPDATE: looking to do a full rewrite and fix lots of bugs, but have limited time. Contact me if you'd like to help out!
Mostly untested, so if you find a bug or have a feature request, let me know!
Note: this only works with .xlsx files. That means .xls files will not work.
Formatting from the Excel document won't be preserved, only the text will be extracted.
Lovingly hacked together by Eric Wood